The present invention relates to gas diffusers for use with centrifugal and axial fans and more particularly to such diffusers for use with lift fans of surface effect ships.
Surface effect ships require that their lift systems should have minimum volume and weight, yet provisions must be made for high static efficiency under conditions of fluctuating demand characteristic of this kind of load. Unfortunately, conventional high efficiency fans of minimum weight and size have high gas flow velocities supplying gas with inordinately high amounts of kinetic energy. These fans ordinarily require the use of large gas diffusers which convert the kinetic energy of the flowing gas into the potential energy of pressure. Furthermore, it is characteristic of both axial and centrifugal fans that they operate at peak efficiency for a relatively narrow range of volume flow. Special difficulties may develop when the flow rate is reduced below some operating point for the machine. Surge or pulsation may result from unstable operating conditions and the machine can be damaged.
One approach for extending the range of volume flow at which a fan will operate at peak efficiency and for lowering the design flow rate point at which surge will occur is to provide an improved diffuser. Diffusers convert kinetic energy in the discharge from the rotor to pressure energy. In fans, as in other rotohydrodynamic machinery, all the energy imparted to the fluid is provided by the rotor, the energy in the air at the exit from the rotor being in the form of both pressure energy and kinetic energy. In a high efficiency fan with a well designed rotor the kinetic component of the total energy is higher than can be practically used. It is the function of the diffuser to convert this kinetic energy into potential energy by slowing down the air in an orderly manner. In the case of centrifugal fans, the air must first be collected in a volute built around the rotor which can additionally function as a diffuser if the cross-sectional area of the volute is made such that the average velocity at any section is less than that leaving the rotor. However, energy losses occur in this volute due to mixing of high and low velocity air if it is used as a diffuser.
A better system for use with centrifugal fans is to place a special chamber between the rotor and volute in which the air is allowed to slow down before entering the volute. This special chamber consists of set of parallel walls in which air leaving the rotor spirals outward in free vortex flow with resulting decrease in velocity at the outer edge of the chamber. In the case of axial flow fans, air leaving the outlet guide vanes of the fan must enter a long conical diffuser in which energy conversion takes place. However, all these diffuser systems suffer from the deficiency that they are bulky and not adjustable to compensate for varying demand flow rates through the fan.
Some adjustable flow rate diffusers which operate to vary the cross-sectional area of the diffuser chamber by sliding part of one wall of the chamber relative to the other wall, as exemplified by embodiments of U.S. Pat. No. 3,478,955, are available for use with centrifugal fans. These devices operate with mixed radial and tangential flow at high velocities, which conditions cause the flow to be unmanageable for varying flow rates and cause high friction and turbulence energy losses. Furthermore, such diffusers do not meet the size, weight, and configuration requirements of surface effect ship lift fan systems nor do they provide the valving action necessary on surface effect ships to completely block water from splashing into the diffuser and fan machinery when the fans are shut down so that the craft is off-cushion.